1. Field of the Invention
The present invention relates to a constant current driving circuit used in an optical communication device, an optical system, and the like.
2. Description of the Related Art
An example of circuit configuration using device characteristics, an example of a high-precision circuit, an example of high-precision and low-power-consumption configuration, and the like are generally used for constant current driving circuits.
(1) Example of Circuit Configuration Using Device Characteristics
FIG. 13A and FIG. 13B are diagrams showing a conventional example of circuit configuration of a constant current driving circuit using device characteristics. FIG. 13A shows an example of configuration using a FET. FIG. 13B shows an example of configuration using a bipolar transistor. As shown in FIG. 13A, the constant current driving circuit has an N-type FET (NFET) 6 and a load 8 connected in series with each other between a power supply 2 and a ground 4. An input voltage 10 is connected between a source and a gate of the NFET 6 to apply a constant source-to-gate voltage, so that a constant current flows from the power supply 2 to the load 8 side.
On the other hand, the constant current driving circuit shown in FIG. 13B has a load 14, an NPN transistor (Tr) 16, and a resistance 18 connected in series with each other between a power supply 10 and a ground 12. An input voltage 20 is connected between a base of the Tr 16 and the ground, so that a constant current flows from the power supply 10 through the load 14, the Tr 16, and the resistance 18 to the ground 12 side.
(2) Example of High-Precision Circuit
FIG. 14 is a diagram showing an example of a high-precision circuit as a conventional constant current driving circuit. As shown in FIG. 14, an NPN transistor (Tr) 24, a load 26, and a resistance 28 are connected in series with each other between a power supply 20 and a ground 22. An input voltage is connected to a plus terminal of a differential amplifier 30, and one terminal of a monitoring resistance 28 having another terminal grounded is connected to a minus terminal of the differential amplifier 30. The differential amplifier 30 applies a control voltage to a base of the transistor 24 such that a load current flowing through the load 26 is a constant current.
(3) Example of High-Precision and Low-Power-Consumption Circuit
FIG. 15 is a diagram showing an example of a high-precision and low-power-consumption circuit as a conventional constant current driving circuit. As shown in FIG. 15, a PFET 44 and a diode 46 are connected in series with each other between a power supply 40 and a ground 42. A coil 48, a load 50, and a monitoring resistance 52 are connected in series with each other between a drain of the PFET 44 and the ground 42. The diode 46 has an anode connected to the ground 42, and a cathode connected to the drain of the PFET 44. A differential amplifier 54 has a minus side connected to one terminal of the monitoring resistance 52, a plus side connected to an input voltage Vin, and an output side connected to a minus side of a comparator 56. A plus terminal of the comparator 56 is connected with an output side of a triangular wave generator circuit 58. The differential amplifier 54 obtains a difference between the input voltage and a voltage corresponding to a driving current. The difference is converted by the triangular wave generator circuit 58 and the comparator 56 into pulse width corresponding to the output voltage level of the differential amplifier 54. The pulse signal is output to a gate of the FET 44. When a high level is applied to the gate of the FET 44, the FET 44 is turned off, and when a low level is applied to the gate of the FET 44, the FET 44 is turned on. The FET 44 is turned off for a time corresponding to the pulse width. When the PFET 44 is turned on, a load current flows from the power supply 40 side through the FET 44, the coil 48, the load 50, and the monitoring resistance 52 to the ground 42.
When the PFET 44 is turned off, a potential at a point of connection of the coil 48 with the FET 44 is decreased to turn on the diode 46. A load current flows from the ground 42 side through the coil 48 to the load 50 side, whereby the load current is smoothed. The pulse signal is applied to the gate of the FET 44 so as to make the load current constant, whereby the load current converges at a constant current.
However, the conventional constant current driving circuits have the following problems. The examples of circuit configuration using device characteristics shown in FIG. 13A and FIG. 13B use individual characteristics of the devices being used which characteristics represent a relation between current and voltage. While the circuit configurations are simple, the circuit configurations have a problem in that the load current varies depending on individual variations and changes in the environment such as temperature and the like. The example of the high-precision circuit shown in FIG. 14 generates a current monitoring voltage by the current monitoring resistance, and supplies a high-precision load current by suppressing dependence on the device being used by negative feedback. Since the control transistor controls the load current by consuming power, the high-precision circuit has a problem of high power consumption by the circuit. The example of the high-precision and low-power-consumption circuit shown in FIG. 15 is also capable of high-precision control by negative feedback as in the example of the high-precision circuit. The high-precision and low-power-consumption circuit is configured to perform pulse control of the control device (FET 44), and thereby the control device performs only switching operation to suppress power consumption of the control device. However, the high-precision and low-power-consumption circuit has a slow response speed and is thus unable to respond quickly.
Thus, in the circuit configuration using device characteristics and the high-precision circuit, the same current as the supply current flows through each transistor. Hence power consumption of a combination of the circuit and the load is Power supply voltage×Supply current. Thus there is a circuit power consumption depending on the power supply voltage in addition to power consumption by the load. This circuit power consumption is unnecessary power consumption. The power consumption of an ideal high-precision and low-power-consumption circuit excluding the load and the monitoring resistance is zero, and thus the high-precision and low-power-consumption circuit is ideal in terms of power consumption. However, since the pulse waveform is smoothed by the diode and the coil, the high-precision and low-power-consumption circuit is not capable of quick response in at least a pulse period or more.